For example, it is common practice to close bottles of liquids by means of a cap that is connected via breakable solid bridges to a ring that is secured to the neck of the bottle. Commonly the cap and the ring are made of metal if the bottle is made of glass, or of polyethylene if the bottle is made of PVC. Nevertheless, such closure systems present problems when recycling bottles. The ring which is secured to the bottle is generally made of a material that is different from that from which the bottle is made and which cannot be recycled under the same conditions as the bottle. The ring must therefore be removed from the bottle prior to recycling, and that increases the cost of recycling.
It is also well known that glass bottles can be closed by means of a crimped metal capsule or "bottle-top", thereby guaranteeing that the bottle has not been opened. However it is then necessary to use a bottle opener in order to open the bottle, and that is not always practical.